Many application environments use analog-to-digital (A/D) and/or digital-to-analog (D/A) conversion to interface real-world systems, which typically involve continuously-varying analog signals, with digital systems for processing the analog signals. One such environment, for example, involves converting speech or other analog sound signals to digital signals for transmission over a telephone line or other communication channel. Converting these analog signals to a digital form significantly improves signal quality and results in a number of other advantages. For example, A/D conversion facilitates encryption of speech, improving privacy of communications.
Several techniques exist for A/D conversion, such as successive approximation, parallel or flash A/D conversion, pulse code modulation (PCM), delta modulation, and sigma-delta modulation. Generally, many of these techniques involve sampling an analog signal at a predetermined rate. The samples are then quantized to discrete levels, from which the A/D converter generates digital values. The discrete levels are spaced at relatively large intervals to reduce the possibility of misinterpretation of the signal levels.
In quantizing the samples to the discrete levels, the A/D converter typically uses amplifiers to magnify the analog signal. This amplification or magnification process involves using a gain current to control the amount of amplification. For example, in environments characterized by small analog signals, the A/D converter uses a fairly large gain factor. On the other hand, in applications in which the analog signals vary over a relatively large range, less amplification is needed. In these applications, the A/D converter uses a small gain factor.
Some application environments involve performing A/D conversion of analog signals received from multiple sources or channels. These signals often have different ranges of variation. Accordingly, it is desirable to use a different gain factor for each channel. In environments in which the signal levels in the individual channels can vary over different ranges, the ability to select a gain factor for each channel from a number of choices is also desirable. Many techniques for providing this flexibility involve using a number of gain-setting components for each channel. These components significantly increase the device and/or board area of the A/D converter and increase manufacturing costs.